/*
 * tinfgzip - tiny gzip decompressor
 *
 * Copyright (c) 2003-2019 Joergen Ibsen
 *
 * This software is provided 'as-is', without any express or implied
 * warranty. In no event will the authors be held liable for any damages
 * arising from the use of this software.
 *
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 *
 *   1. The origin of this software must not be misrepresented; you must
 *      not claim that you wrote the original software. If you use this
 *      software in a product, an acknowledgment in the product
 *      documentation would be appreciated but is not required.
 *
 *   2. Altered source versions must be plainly marked as such, and must
 *      not be misrepresented as being the original software.
 *
 *   3. This notice may not be removed or altered from any source
 *      distribution.
 */

#include "tinf.h"

typedef enum {
  FTEXT = 1,
  FHCRC = 2,
  FEXTRA = 4,
  FNAME = 8,
  FCOMMENT = 16
} tinf_gzip_flag;

static unsigned int read_le16(const unsigned char *p) {
  return ((unsigned int)p[0]) | ((unsigned int)p[1] << 8);
}

static unsigned int read_le32(const unsigned char *p) {
  return ((unsigned int)p[0]) | ((unsigned int)p[1] << 8) |
         ((unsigned int)p[2] << 16) | ((unsigned int)p[3] << 24);
}

int tinf_gzip_uncompress(void *dest, unsigned int *destLen, const void *source,
                         unsigned int sourceLen) {
  const unsigned char *src = (const unsigned char *)source;
  unsigned char *dst = (unsigned char *)dest;
  const unsigned char *start;
  unsigned int dlen, crc32;
  int res;
  unsigned char flg;

  /* -- Check header -- */

  /* Check room for at least 10 byte header and 8 byte trailer */
  if (sourceLen < 18) {
    return TINF_DATA_ERROR;
  }

  /* Check id bytes */
  if (src[0] != 0x1F || src[1] != 0x8B) {
    return TINF_DATA_ERROR;
  }

  /* Check method is deflate */
  if (src[2] != 8) {
    return TINF_DATA_ERROR;
  }

  /* Get flag byte */
  flg = src[3];

  /* Check that reserved bits are zero */
  if (flg & 0xE0) {
    return TINF_DATA_ERROR;
  }

  /* -- Find start of compressed data -- */

  /* Skip base header of 10 bytes */
  start = src + 10;

  /* Skip extra data if present */
  if (flg & FEXTRA) {
    unsigned int xlen = read_le16(start);

    if (xlen > sourceLen - 12) {
      return TINF_DATA_ERROR;
    }

    start += xlen + 2;
  }

  /* Skip file name if present */
  if (flg & FNAME) {
    do {
      if (start - src >= sourceLen) {
        return TINF_DATA_ERROR;
      }
    } while (*start++);
  }

  /* Skip file comment if present */
  if (flg & FCOMMENT) {
    do {
      if (start - src >= sourceLen) {
        return TINF_DATA_ERROR;
      }
    } while (*start++);
  }

  /* Check header crc if present */
  if (flg & FHCRC) {
    unsigned int hcrc;

    if (start - src > sourceLen - 2) {
      return TINF_DATA_ERROR;
    }

    hcrc = read_le16(start);

    if (hcrc != (tinf_crc32(src, start - src) & 0x0000FFFF)) {
      return TINF_DATA_ERROR;
    }

    start += 2;
  }

  /* -- Get decompressed length -- */

  dlen = read_le32(&src[sourceLen - 4]);

  if (dlen > *destLen) {
    return TINF_BUF_ERROR;
  }

  /* -- Get CRC32 checksum of original data -- */

  crc32 = read_le32(&src[sourceLen - 8]);

  /* -- Decompress data -- */

  if ((src + sourceLen) - start < 8) {
    return TINF_DATA_ERROR;
  }

  res = tinf_uncompress(dst, destLen, start, (src + sourceLen) - start - 8);

  if (res != TINF_OK) {
    return TINF_DATA_ERROR;
  }

  if (*destLen != dlen) {
    return TINF_DATA_ERROR;
  }

  /* -- Check CRC32 checksum -- */

  if (crc32 != tinf_crc32(dst, dlen)) {
    return TINF_DATA_ERROR;
  }

  return TINF_OK;
}
